1. Field of the Invention
The present invention is related to forming wire bonds between the contact pads on semiconductor devices and individual lead frame fingers of a lead frame.
More specifically, the present invention is related to the apparatus and method of supporting the lead fingers of a lead frame during a wire bonding process using a support arm with a lead support portion that is positionable between the lead fingers and the die prior to the bonding process to help substantially stabilize the lead fingers during the bonding process.
2. State of the Art
Well known types of semiconductor devices are connected to lead frames and subsequently encapsulated in plastic for use in a wide variety of applications. Typically, the lead frame is formed from a single continuous sheet of metal by metal stamping operations. In a conventional lead frame, the lead frame includes an outer supporting frame, a central semiconductor chip supporting pad and a plurality of lead fingers, each lead finger having, in turn, a bonding portion thereof near the central chip supporting pad. Ultimately, the outer supporting frame of the lead frame is removed after the wire bonds between the contact pads of the semiconductor chip device and the lead fingers are made and the semiconductor device and a portion of the lead frame have been encapsulated.
In the assembly of semiconductor devices utilizing such conventional lead frames, a semiconductor die is secured to the central supporting pad (such as by a solder or epoxy die-attach, although a double-sided adhesive tape-type attach has also been suggested in the art) and then the entire lead frame, with the semiconductor die thereon, is placed into a wire bonding apparatus including a clamp assembly for holding the lead frame and die assembly, and clamping the lead fingers for bonding.
In contrast to a conventional lead frame, U.S. Pat. No. 4,862,245 issued Aug. 29, 1989 to Pashby et al. illustrates a so-called "leads over chip" arrangement ("LOC") on the semiconductor die. A plurality of lead fingers of the lead frame extend over the active surface of a semiconductor die toward a line of bond pads thereon wherein bond wires make the electrical connection between the lead fingers and the bond pads. An alpha barrier such as a polyamide tape (for example, Kapton.TM. tape) is adhered between the semiconductor die and the lead fingers. This configuration, which eliminates the use of the previously-referenced central die attach pad, may assist in limiting the ingress of corrosive environment contaminants after encapsulation of the semiconductor device, achieve a larger portion of the lead finger path length encapsulated in the packaging material, and reduce electrical resistance caused by the length of the bond wires (i.e. the longer the bond wire, the higher the resistance) and potential wire sweep problems in the encapsulation of the semiconductor device aggravated by long wire loops.
In a standard wire bonding process, the bond wires are attached, one at a time, from each bond pad on the semiconductor device and to a corresponding lead finger. The bond wires are generally attached through one of three industry-standard wire bonding techniques: ultrasonic bonding--using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bonding--using a combination of pressure and elevated temperature to form a weld; and thermosonic bonding--using a combination of pressure, elevated temperature, and ultrasonic vibration bursts.
To form a good bond during the wire bonding processing, it is preferable to perform the bonding at an elevated and somewhat stable temperature. Therefore, as noted above, the lead frame assembly including the attached semiconductor die is generally placed on a heater block. The semiconductor die is then clamped (via the lead frame) to the heater block by a clamping assembly. With a conventional lead frame, the lead fingers are clamped directly against the underlying heater block. Whereas, in an LOC lead frame, the lead fingers are biased between the clamp and the active surface of the semiconductor die heater block. Thus, in an LOC lead frame arrangement, the clamping assembly and bonding apparatus apply pressure against the die thereby causing possible damage. In addition, heating of the lead fingers in an LOC lead frame for wire bonding must be done through heating the die as apposed to directly heating the lead fingers by the heater block in a conventional lead frame.
Therefore, in an LOC lead frame configuration it would be advantageous to develop an apparatus to prevent the clamping assembly and bonding apparatus from applying force against the die. In addition, it would be advantageous to develop an apparatus for transferring heat directly from the heat block to the lead fingers.
In an LOC structure, the Kapton.TM. tape comprising the alpha barrier or dielectric between the semiconductor and the lead fingers becomes soft at the elevated temperature. The softening of the tapes allows the lead fingers and/or semiconductor die to move in response to ultrasonic energy or pressure (force) exerted by the wire bonding head (capillary). As a result, the mechanical integrity of the wire bond to the lead fingers is diminished. Furthermore, a "bouncing" motion is imparted to the lead fingers by the wire bonding head movement, which motion may be exacerbated by the heat softened tape. This bouncing motion can also result in poor wire bonds which subsequently fail.
Thus, die fabricators are somewhat compelled to select the die attach compound (or other means) and alpha barrier tape based on the thermal stability of the materials rather than on the basis of the most effective material for a given application.
Therefore, it would be advantageous to develop an apparatus that would replace the alpha barrier tape while stabilizing the semiconductor die and the lead fingers during the wire bonding process.
Typical apparatus and methods for clamping the lead frame during the wire bonding process or for clamping and advancing the lead frame are illustrated in U.S. Pat. Nos. 4,765,531, 5,082,165, 5,238,174, 5,264,002, 5,307,978, 5,322,207, and 5,372,972. However, such apparatus and methods do not address the problem of supporting the lead fingers during the wire bonding process or preventing the application of force on the die.
Such prior art apparatus and methods have been directed at advancing and orienting the lead frame but have not attempted to solve the problems of forming reliable wire bonds between the contact pads of semiconductor devices and lead fingers of lead frames.
There have been other attempts to overcome the problem of the bouncing motion imparted to the lead fingers by the wire bonding head movement. For example, for bonding LOC structures, rigid clamping plates having bond site windows therein have been reconfigured so that the bond site window is reduced in size and the downwardly-extending lip or periphery contacts the lead fingers extending over the die and clamps the lead fingers directly thereto. However, the rigid clamp has been found to be too rigid and unyielding for use with an LOC configuration, and may possibly damage the die. Moreover, the use of a rigid clamp adds to the force exerted against the die and does nothing to prevent the application of force by the bonding apparatus.
The present invention is directed to an improved wire bonding apparatus and method for forming such wire bonds.